The ectopic expression of cDNAs in mammalian cells has been widely used to isolate genes encoding growth factors and their receptors. Transfection of a mouse testis cDNA library into NIH/3T3 fibroblasts resulted in the isolation of several cDNA clones with high titer transforming capacity. The sequence of one of these clones. Tst. a 1.6 kB cDNA with an open reading frame encoding a protein of 208 amino acids. revealed that Tst was the eighth member of the fibroblast growth factor (FGF) gene family. Conditioned medium from NIH/3T3 cells expressing Tst stimulated DNA synthesis in fibroblasts. epithelial cells and glial cells. indicating a broad spectrum of mitogenic activity. The Tst cDNA detected a 1.6 kB mRNA in RNA isolated from adult mouse testis. but not in RNA isolated from mouse liver. brain. heart or kidney. Ribonuclease protection analysis demonstrated that the abundance of Tst mRNA was highest during embryonic day 11 and 13 of mouse embryonic development. In addition. a partial human FGF-8 cDNA was isolated by screening a human testis cDNA library with the Tst cDNA.